The invention relates to digital voice enhancement, DVE, communication systems, and more particularly to improvements enabling increased gain.
The invention is applicable to DVE systems, including duplex systems, for example as shown in U.S. Pat. No. 5,033,082, and in U.S. application Ser. No. 08/927,874, filed Sep. 11, 1997, simplex systems, for example as shown in U.S. application Ser. No. 09/050,511, filed Mar. 30, 1998, all incorporated herein by reference, and other systems. The DVE communication system includes a first acoustic zone, a second acoustic zone, a microphone at the first zone, and a loudspeaker at the second zone and electrically coupled to the microphone such that the speech of a person at the first zone can be heard by a person at the second zone as transmitted by an electrical signal from the microphone to the loudspeaker.
Speech signals tend to have large peak-to-rms (root mean square) ratios, which define the signal levels permissible before hardware clipping, and the energy levels contained within those signals. Often when processing speech signals it is desired for more rms energy at the output of the signal path. This is usually accomplished by increasing the signal gain in a linear fashion. At some limit the speech signals can no longer have gain added to them, and they will clip analog electronics and/or saturate numerical processes.
In the present invention, the dynamic range of the electrical signal from the microphone is altered, to effectively allow more gain to be added to small signals increasing the rms level while limiting the peaks of the large signals by adding less gain preventing hard clipping. Nonlinear and/or differential gain is applied. In one embodiment, a slope intercept formula is used to calculate gain of the speech input signal when the signal passes a designated threshold, which formula is preferably a soft clipping algorithm. If the speech input signal is less than the threshold, a unity linear gain is applied, and the signal passes through the process unaffected. Typically a net gain is added downstream of the soft clipping process before transmission to the loudspeaker. The system allows the gain to be increased for low level signals, e.g. soft talkers, and limited for high level signals, e.g. loud talkers. The system reduces the problem of talker level dependency for the effectiveness of the DVE system.